Degenerate chemical shift

NOEs that involve groups of protons with degenerate chemical shifts, in particular methyl groups, may be referred to pseudoatoms located in the center of the protons that they represent, and the upper bound is increased by a pseudoatom correction equal to the proton-pseudoatom distance [27, 28]. Another method that usually incurs a smaller loss of information [29] is to treat NOEs for groups of protons with degenerate chemical shifts as ambiguous distance constraints (see Eq. (13) below). 

The l3C NMR spectrum of the C4H7+ cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species (13). A small temperature dependence of the l3C chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species (13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl (14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

The larger the protein, the greater the resonance overlap. Often several different residues have degenerate 13Ca frequencies which will make an unambiguous assignment difficult or even impossible. In these cases, additional information from different NMR experiments is required. One possibility is to use the carbonyl chemical shift instead of the Ca chemical shift and measure the HNCO/HN(CA)CO pulse sequence pair [37, 45, 46, 49, 50]. As with the HNCA/HN(CO)CA combination, one of the experiments, the... 

Timberlake and coworkers have studied the degenerate rearrangement of pentacyclo-propylethyl cation 56 (involving 1,2-cyclopropyl shifts) under long-lived stable ion conditions81 82 (equation 39a). The rearrangement could not be frozen even at -80 °C. However, additivity of 3C NMR chemical shift analysis7 indicates the classical trivalent nature of the carbocation. 

The 3-homonortricyclyl cation 70 was prepared by the isomerization of bicyclo[3.2.1]oct-3-en-2-yl cation 71 at 20 °C in SbFs/SC ClF solution83. The ion shows a threefold degenerate rearrangement between -85 °C to 20 °C. At 20 °C the C4, C6, C8 and Cl, C3, C7 carbons become equivalent with an average of 36.19 ppm and 135.8 ppm, respectively (equation 43). Below -80 °C the cation is a static secondary cyclopropylcarbinyl cation with the cationic center chemical shift at <5I3C 234.1. 

In D4h (tetragonal) symmetry there is only one unpaired electron in two degenerate orbitals of correct symmetry to give rise to n bonds with a n orbital of the porphyrin moiety [34]. The H NMR spectrum of Fe(protoporphyrin IX)-imidazole-cyanide is reported in Fig. 5.17 [35]. The free rotation of the imidazole ring about the metal-nitrogen bond, which is fast on the NMR timescale, simulates a tetragonal symmetry as far as the chemical shifts are concerned [36]. The four methyls are all downfield, though to a quite smaller value than in the case of... 

In a study of rates of degenerate 1,2-shifts in tertiary carbocations, Saunders and Kates854 used higher-field (67.9 MHz) 13C NMR line broadening in the fast-exchange limit. The 2-butyl cation showed no broadening at — 140°C. Assuming the hypothetical frozen out chemical shift difference between C(2) and C(3) to be 227 ppm, an upper limit for AG was calculated to be 2.4 kcal mol 1. 

Torocheshnikov (88) has studied the temperature variation of 1I9Sn chemical shifts in two trimethylstannyl derivatives of cyclopentadiene, [10] and [11]. In these two compounds, a non-degenerate metallotropic... 

Some of the CH2 crosspeaks are degenerate, meaning that the two protons have the same ll chemical shift (d and r). This can be a coincidence, but it is more likely to happen in a flexible chain, so we would suspect carbons 22-24 in cholesterol, although 22 is less likely because it is next to a chiral center. 

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